Integration into the bacterial chromosome

The small pieces of DNA known as plasmids, which replicate independently of the chromosomes, have been discussed briefly in Chapter 5. Plasmids share a number of properties with viruses, and both are important to the techniques of contemporary molecular biology and genetic engineering. Bacterial plasmids may be present as one or several copies for each chromosome. Episomes are plasmids that are able to become integrated into the bacterial chromosome. Some extrachromosomal elements are episomes in one host and plasmids in another. Bacterial... 

Episomes Plasmids that can imdergo integration into the bacterial chromosome. 

Integration of the prophage into the bacterial chromosome ensures that, on cell division, each daughter cell will acquire the set of viral genes. 

Figure 2. Integration of the IS50L-B.t. element into the bacterial chromosome. A. A plasmid carrying IS50L-B.t. and IS50R, enters the cell via conjugation. B. The IS50L-B.t. element transposes into the chromosome by in cis complementation mediated by IS50R. C. Only the IS50L-B.t. remains in the cell.

Prophage An inactive phage genome that is present in a bacterial cell and its progeny. It is integrated into the host chromosome. 

When an Hfr strain conjugates with an F (female), replication of the entire male chromosome commences at some point near the end of the integrated F agent, and genes of the bacterial chromosome followed by those of the F factor are transferred into the female. Only a single strand of DNA (customarily referred to as the plus strand) is transferred from the donor cell and into the recipient cell (Fig. 26-3). There the complementary minus strand is synthesized to form a complete double-stranded DNA molecule bearing the genes from the Hfr cell. Only rarely does a copy of the entire chromosome of the donor cell enter the female cell. More often the DNA strand, or perhaps the pilus itself, breaks and only part of the chromosome is transferred. 

Figure 26-3 Integration of an F agent into a bacterial chromosome and transfer of some bacterial genes into another cell. (A) Incorporation of the F agent into E. coli genome and transfer of the "plus" strand of DNA out to a female recipient cell. (B) Genetic recombination between a piece of transferred DNA and the genome of the recipient cell.

After a viral particle absorbs onto the surface of the bacterial cell, the viral genome is injected into the cell. If the virally infected cell enters the lytic phase, its molecular machinery begins synthesis of the components of new virus. If, instead, the viral genome is integrated into the host cell chromosome, the cell enters the lysogenic phase. At a later time the cell may enter the lytic phase. 

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